U.S. patent application number 10/320936 was filed with the patent office on 2004-06-17 for radio frequency nmr resonator with split axial shields.
Invention is credited to de Swiet, Thomas, Mehr, Knut G..
Application Number | 20040113617 10/320936 |
Document ID | / |
Family ID | 32506996 |
Filed Date | 2004-06-17 |
United States Patent
Application |
20040113617 |
Kind Code |
A1 |
de Swiet, Thomas ; et
al. |
June 17, 2004 |
Radio frequency NMR resonator with split axial shields
Abstract
An NMR probe comprises a resonator formed of two separate
conducting loops disposed on opposite sides of the sample, where
one loop is driven and the other floats in a preferred operating
mode. Slotted shields are disposed coaxially within said loops and
outside the sample with slots aligned with gaps between the
loops.
Inventors: |
de Swiet, Thomas; (Redwood
City, CA) ; Mehr, Knut G.; (San Francisco,
CA) |
Correspondence
Address: |
Varian Inc.
Legal Department
3120 Hansen Way D-102
Palo Alto
CA
94304
US
|
Family ID: |
32506996 |
Appl. No.: |
10/320936 |
Filed: |
December 17, 2002 |
Current U.S.
Class: |
324/318 ;
324/300; 324/303; 324/322 |
Current CPC
Class: |
G01R 33/34092 20130101;
G01R 33/343 20130101; G01R 33/34069 20130101 |
Class at
Publication: |
324/318 ;
324/322; 324/300; 324/303 |
International
Class: |
G01V 003/00 |
Claims
What is claimed is:
1. An NMR probe comprising a resonator structure comprising first
and second conductors forming current supporting loops, each said
loop comprising a pair of terminals, said loops disposed on
opposing facing portions of the lateral surface of a cylindrical
volume, each said loop defining an aperture on respective portions
of said lateral surface, said first and second loops separated by
first and second gaps on said lateral surface.
2. The NMR probe of claim 1 further comprising a pair of axially
displaced cylindrical shells, each said shell disposed coaxially
within said resonator, each said shell comprising an axial slot
therein, oppositely facing edges of said shells in substantial
axial alignment with axial inner edges of said loops.
3. The NMR probe of claim 2 wherein said slots are in substantial
azimuthal alignment with said gaps.
4. The NMR probe of claim 3 further comprising a dewar for
containing said resonator and comprising an inner wall for
receiving said sample at ambient temperature and maintaining said
resonator and said shields at a cryogenic temperature.
5. The NMR probe of claim 4 wherein the inner surface of the inner
wall of said dewar supports a pair of conducting cylinders in
substantial axial alignment with said shields.
6. The NMR probe of claim 3 further comprising a pair of conducting
cylinders, said conducting cylinders symmetrically disposed on said
axis and within and spaced from said cylindrical shells, said
conducting cylinders adapted to receive a sample on the axis
thereof.
7. The NMR probe of claim 5 or 6 wherein said conducting cylinders
exhibit relatively poor conductivity, whereby eddy currents induced
on the surfaces thereof are rapidly attenuated.
8. The method of operating an NMR probe comprising a resonator
structure comprising first and second loops disposed on opposite
sides of said sample, each said loop comprising a pair of terminals
precluding closure of said loop, said method comprising the steps
of driving the first said loop through said terminals by applying
RF power thereto, floating the terminals of said second loop and
inductively coupling said second loop to said first loop.
9. An NMR system for study of a sample, comprising a magnet for
supplying a homogeneous magnetic field, an RF source and an RF
detector and an NMR probe disposed within said magnetic field and
communicating selectively with said RF source and said RF detector,
said probe comprising a resonator structure comprising first and
second conductors forming current supporting loops, each said loop
comprising a pair of terminals, said loops disposed on opposing
facing portions of the lateral surface of a cylindrical volume,
each said loop defining an aperture on respective portions of said
lateral surface, said first and second loops separated by first and
second gaps on said lateral surface.
10. The apparatus of claim 9 wherein each said loop comprises a
pair of terminals and the resulting four said terminals are
accessible for communication with an RF source in a variety of ways
to implement corresponding distinct circuit properties.
11. The apparatus of claim 9 wherein one said loop is driven and
the other said loop is electrically floating.
Description
FIELD OF THE INVENTION
[0001] The invention is in the field of NMR and relates
particularly to RF probe structures.
BACKGROUND OF THE INVENTION
[0002] The central components of a modern high resolution liquid
sample NMR probe include, at least one resonator for coupling RF
(resonant) radiation to (at least one) resonating aggregation of
nuclear spins of a sample. The sample is typically of elongate
extension along an axis coincidental with the direction of the
static polarizing magnetic field B.sub.0. The resonator imposes on
the sample an RF magnetic field (B.sub.1) transverse to B.sub.0.
The achievable homogeneity of B.sub.0 is spatially limited and the
practicality of coupling RF power to the resonator through finite
leads motivate the use of an RF shielding structure interposed
between the leads of the resonator and the sample. The RF shield
structure ideally limits RF coupling to the resonant spins located
within a prescribed axial region of B.sub.0 homogeneity. In
particular excitation of sample portions outside the desired region
of carefully shimmed B.sub.0 homogeneity due to irradiation from
the leads is a parasitic effect to be minimized by the shields.
[0003] The RF coil and shielding is subject to the deleterious
effects of eddy currents arising from rapidly switched independent
magnetic fields, such as magnetic gradient fields. Eddy currents
induced in the coil and shields produce transient magnetic fields
in opposition to the switched field inducing the eddy current.
Inasmuch as these parasitic fields are particularly close to the
sample, the B.sub.0 field homogeneity is degraded and the parasitic
fields, with undesirable persistence, also degrade the relative
timing of steps associated with a given NMR experiment.
[0004] In order to control/reduce eddy currents in RF coil and
shield structures operating at room temperature, the prior art has
incorporated slots into these conductors. See, for example, U.S.
Pat. Nos. 6,008,650; 5,192,911; and WO 92/17799 all commonly
assigned with the present invention. To similar effect, see U.S.
Pat. No. 4,875,013. It should be appreciated that the eddy current
problem is many times more deleterious with the RF resonator at
cryogenic temperatures than that experienced with the RF resonator
at room temperature. For the purpose of this work cryogenic
temperature shall be understood to include temperatures
substantially below ambient. Recent advances in NMR include very
high Q probes operating at rather low temperature. Under such
conditions, eddy currents effects are enhanced and their consequent
deleterious effects require more rigorous suppression.
SUMMARY OF THE INVENTION
[0005] As employed herein, the RF resonator of saddle coil geometry
is completely divided between longitudinally adjacent inductive
members to provide two electrically separate loops disposed on
opposite facing surfaces enclosing the sample space. Each loop,
defining a window to the sample encompasses slightly less then
2.pi. (around the loop) to accommodate the two leads of each loop.
The leads from both loops are disposed longitudinally in the same
direction away from the central region (windows) of the loops. In
the preferred embodiment the coil is excited by application of RF
power to one loop with mutual induction coupling to symmetrically
excite the opposite loop.
[0006] The shields comprise cylindrical conduction each slotted to
provide azimuthal shield portions approaching .pi. in angular
extent. These shield cylinders are coaxial with the RF coil with
the inner axial extent preferably aligned with corresponding outer
edges of the RF coil windows. The separate gaps between the loops
of the RF coil are preferably aligned with the slots of the shields
to provide transverse windows where double resonance experiments
are contemplated.
[0007] The subject matter of the present application is
particularly pointed out and distinctly claimed in the concluding
portion of this specification. However, both the organization and
method of operation, together with further advantages and objects
thereof, may best be understood by reference to the following
description taken in conjunction with accompanying drawings wherein
like reference characters refer to like elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows an NMR system incorporating the invention.
[0009] FIG. 2a shows a planar mapping of an RF coil of prior
art.
[0010] FIG. 2b shows a planar mapping of RF shields for the prior
art of FIG. 2a.
[0011] FIG. 3a shows a planar mapping of the RF coil of the
invention.
[0012] FIG. 3b shows the planar mapping of the RF shields for the
RF coils of FIG. 3a.
[0013] FIG. 3c illustrates the equivalent circuit for the preferred
mode of operation.
[0014] FIG. 3d shows a prior art cryogenic RF coil and shields for
a comparative test purpose.
[0015] FIG. 4 is a perspective view of the embodiment of FIGS. 3a
and 3b.
[0016] FIG. 5 shows eddy current responsiveness performance of the
invention.
[0017] FIG. 6 illustrates another embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0018] FIG. 1 represents the context of the invention represented
by schematicised NMR instrument. An acquisition/control processor
110 communicates with an RF source 112, modulator 114 and RF
receiver 116, including analog-to-digital convertor 118 and a
further digital processor 120. The modulated RF power irradiates an
object/sample 123 in a magnetic field 121 through a probe 122 and
response of the sample/object is intercepted by probe 122
communicating with receiver 116. The response typically takes the
form of a transient time domain waveform or free induction decay.
This transient waveform is sampled at regular intervals and the
samples are digitized in adc 118. The digitized time domain
waveform is then subject to further processing in processor 120.
The nature of such processing may include averaging the time domain
waveform over a number similar such waveforms and transformation of
the averaged time domain waveform to the frequency domain yields a
spectral distribution function directed to output device 124.
Alternatively, this procedure may be repeated with variation of
some other parameter, and the transformation(s) from the data set
may take on any of a number of identities for display or further
analysis.
[0019] FIG. 2a is representative of (room temperature) prior art
slotted resonator structures 12 and FIG. 2b similarly depicts the
shielding structure 14 interposed between a sample and the
resonator. These figures are planar mappings of the cylindrical
forms in use wherein the resonator and shield are understood to
exhibit radii a and b respectively where a>b. Arrows within FIG.
2a indicate the instantaneous RF current direction in an embodiment
where RF power is applied between terminal portions 16 and 17.
Windows 18 and 19 define the two current loops of the coil. Arrows
on the several conductor portions suggest the instantaneous current
direction. RF magnetic flux (B.sub.1) is ideally distributed
uniformly through the area of these oppositely facing windows.
Slots 20a and 20b (comprising half slots 20b" and 20b') serve to
define electrical structure of the resonator structure 12. Prior
art has employed RF shields taking the form of cylindrical shell
conductors, axially flush with the inner edge of the RF window and
extending axially outward to shield portions of the sample (distal
in relation to the windows) from unwanted excitation. These shields
support eddy currents due to rapidly switched gradient fields. The
shield structure 14 may be briefly described as a pair of slotted
rings symmetrically aligned with windows 18 and 19 and with slots
21a and 21b (comprising 21b' and 21b") similarly aligned with
resonator slots 20a and 20b. The shield structure is typically
electrically floating and serves to shield the leads 16 and 17 from
the sample and to shield the sample portion remote from the window
regions from excitation. This limits sample excitation to the axial
region projected from the windows 38 and 39, a region of
homogeneity of both the polarizing field and the RF field. As thus
described, the prior art may be more closely identified with U.S.
Pat. No. 6,008,650 as representative prior art.
[0020] Turning now to FIGS. 3a and 3b there is shown the preferred
embodiment of the invention wherein the resonator 32 comprises two
completely distinct current loops surrounding windows 38 and 39.
One said loop is excited from leads 36 and 37 and the other loop is
excited wholly through mutual inductive coupling to the first loop.
Balance and electrical symmetry of the two loops is preserved
through maintenance of geometrical symmetry: that is, the phantom
(floating) leads 34 and 35 of the inductively coupled loop present
similar capacity contribution to loop surrounding window 39 as do
the leads 36 and 37 to the driven loop surrounding window 38. In
addition to the electrical symmetry thus served, the geometrical
symmetry aids in avoiding magnetic inhomogeneties in the polarizing
field.
[0021] The shield structure 34 presents slot 41a and slot 41b
(comprising half slots 41b' and 41b"). Further, the capacitance
furnished to each loop is adjusted by indented regions 42. It
should be recognized that the slots 41a and 41b in the shield 34,
together with corresponding slots 40a and 40b in the resonator form
another pair of windows open to the sample. (To preserve
nomenclature, the word "slot" is here synonymous with "gap".) These
slot windows are oriented orthogonal to the prevailing azimuthal
orientation of the windows 38 and 39. These slot windows are
convenient for double resonance experiments involving a second
coaxial resonator disposed externally to resonator 32. The second
resonator illuminates the sample through the aperture formed by the
gaps between the loops of resonator 32, wherein two independent
resonance conditions are concurrently available with orthogonal
directions for the corresponding RF magnetic fields.
[0022] FIG. 4is a perspective view of the resonator and shields of
FIGS. 3a and 3b. Corresponding portions of the perspective figure
bear the same labels as the planar mappings of FIGS. 3a and 3b.
[0023] One quantitative measure of the efficacy of the invention is
the recovery time after a sharp magnetic impulse such as provided
by a rapidly switched magnetic gradient field. FIG. 5 compares the
response of the present invention with a prior art resonator
intended for a cryogenic environment. This prior art is similar to
FIG. 2a adapted for cryogenic operation, for example in a Varian
"Chili.TM. type NMR probe as further shown in FIG. 3d for
performance comparison with FIG. 3a. The two resonator structures
(present invention and prior art), constructed from identical
materials are characterized by similar dimensions. It is apparent
that the FIG. 3d device is a two terminal slotted resonator with
unslotted shields. In contrast the inventive device features full
separated current loops, slotted shields and only a single loop is
driven (mode a of Table 1 below). For this test, the two resonators
were each subject to a magnetic field pulse about 1 ms in width
supplied by a surrounding gradient coil of about 30 (gauss/cm). The
measured time for recovery to 90% of full NMR amplitude for the
resonator of this invention (curve 60) was (about) 250 .mu.sec
compared to about 2.5 ms for the prior art (curve 62). It is
apparent that the example represents an improved recovery time of
about a factor of 10 compared to the representative prior art.
[0024] Table 1 is a tabular summary illustrating the several
modalities supported by the slotted/gap resonator structure of FIG.
2b through connection of the terminals 34,35 of loop 39, and
terminals 36 and 37 of loop 38.
1TABLE 1 Loop configuration jumpers driven terminals a1) Loop 38
driven, loop 39 floats RF dipole field N/A 36 + 37 or, a2) loop 39
driven, loop 38 floats N/A 34 + 35 b) loops in series/same helicity
RF dipole field 34.fwdarw.36 37 + 35 c) loops in series/opposite
helicity RF gradient field 34.fwdarw.37 35 + 36 d) loops in
parallel/same helicity RF dipole field 34.fwdarw.37, 35.fwdarw.36
jumpers e) loops in parallel/opposite helicity RF gradient field
36.fwdarw.34, 37.fwdarw.35 jumpers
[0025] In modes a), terminals 36 and 37 are driven while terminals
34 and 35 float (or vice versa). An homogeneous RF dipole field is
excited with axis principally through the (large) windows 38 and
39. In the mode b, terminal 36 (or 37) is excited together with
terminal 34 (or 35) to produce an RF resonance at a lower frequency
than the a) modes corresponding to the larger inductance of the
series combination of the conductors.
[0026] Another operational possibility (d in table 1) is
established by exciting both loops in phase by driving the two
loops in parallel such that adjacent inductive axial members of the
opposite facing loops support instantaneous RF currents in the same
sense while circulating on the respective loops to produce the
principal resonance. [Capacitive coupling between loops across the
gaps 40a and 40b (at the opposite end from the terminals) supports
a circulating RF current component around gaps 40a 40b to produce
another resonance usually positioned at much higher frequency than
the main resonance.] This choice is illustrated in FIG. 3a by the
dotted lines representing jumpers 44. This operation (and any
jumpering between terminals) restores (to some extent) a path for
eddy currents that was removed by the major slots 40a and 40b
separating the resonator into two distinct loops. It has been found
that this operational approach conveniently facilitates maintenance
of a deuterium lock where attenuated eddy current effects are
tolerable.
[0027] In another operational mode, the corresponding terminals of
the respective loops may be connected serially in opposite
helicity, e.g., adjacent axial inductive members of the loops
support opposite sense of instantaneous RF currents. Similarly,
parallel combination of the loops 38 and 39 in opposite helicity
may be selected. These operational modes produce a radial RF
magnetic field gradient, of interest in certain specialized
experiments.
[0028] It should be apparent that the several operational modes
represented in table 1 would not support resonant operation at the
same frequency. For example direct coupling of the coil loops in
series will not exhibit the same lumped inductance as where the
same loops are connected in parallel. The connection of the
opposite facing loops in the same helicity will not present the
same mutual inductive coupling as where the same loops are
connected in opposite helicity.
[0029] Turning to FIG. 6, there is shown another embodiment of the
invention particularly suited to cryogenic NMR probe structure
wherein the RF coil 32 and shields 34 are disposed in a vacuum
space at cryogenic temperature. The inner surface of the vacuum
enclosure is the outer (radial) surface of a tubular structure 52,
which supports a pair of RF inner shields 54. These inner shields
54 preferably comprise about one mil Cu, e.g., large compared to
the skin depth at operational resonance frequencies. These shield
members 54, centered on window 38 and 39 are axially separated by a
distance in the range of 100% to 200% of the axial window
dimensions. The length of shields 54 is not critical. These inner
shields serve to further reduce excitation of unwanted resonances
from the sample region axially displaced from the window region.
For the cryogenic case, these unslotted shield members 54 are at an
intermediate temperature, typically, close to ambient because a
relatively poor conductivity for these shields is desirable to
better reduce the attendant eddy currents. Alternately, these
innermost shields might be implemented from a relatively poorly
conductor, e.g., an alloy, or these shields might be extremely
thin.
[0030] Although this invention has been described with reference to
particular embodiments and examples, other modifications and
variations will occur to those skilled in the art in view of the
above teachings. It should be understood that, within the scope of
the appended claims, this invention may be practiced otherwise than
as specifically described.
* * * * *